This invention relates to eliminating defects caused during electrographic printing, and more particularly to reducing flaring in an electrographic printing environment.
Electrographic marking, or printing, upon an image recording medium comprises a two-stage process. First, ions are created by electrical breakdown of air in a gap between writing nibs and the recording medium, and then ions (usually negative) are conducted to selected image pixel locations to form an electrostatic image on the recording medium. Second, the electrostatic image is made visible by "toning", which usually involves the passing of the recording medium, bearing the nonvisible, electrostatic image, into contact with a liquid solution containing positively charged dye particles in colloidal suspension. The dye particles are attracted to the negative charge pattern and the density of the dyed image will be proportional to the potential or charge density on the medium.
Two types of recording media in common usage are paper and film. For paper, the bulk is treated to make it conductive and a dielectric layer of about 0.5 mil thick is coated upon its image bearing side. For film media, a substrate such as Mylar.RTM., is given a very thin conductive layer and an overcoat dielectric layer upon its image bearing side.
In the electrographic printing process, electrical contact must be made to the conductive layer of the medium in order to charge the dielectric layer with the electrostatic image. In the case of paper, this is accomplished by direct electrical contact with the backplates of the writing device to the "backside" of the base paper. In the case of film, conductive edge stripes pass through the dielectric layer to the conductive layer providing electrical paths to the conductive layer. Electrical contact is made to the conductive layer through these stripes.
In the process, there must also be a means for establishing the electrical potential difference between the conductive layer and the nibs sufficient for electrical breakdown of the air. In the case of paper, the potential of the conductive base is established by pulsing a backplate which has resistive/capacitive coupling to the back of the medium. In the case of film, the potential of the conductive layer is also established by pulsing the backplate, which in this case is only capacitively coupled to the conductive layer through the Mylar.RTM. base. The latter is shown in U.S. Pat. No. 4,424,522 to Lloyd et al., assigned to the same assignee as this application, and hereby incorporated by reference. Also, refer to U.S. Pat. No. 4,254,424 to Landheer et al., assigned to the same assignee as this application, and hereby incorporated by reference, which discloses an electrostatic recording device wherein a latent electrostatic image is recorded on a dielectric coated sheet by a stylus array spaced apart from the sheet to form an ionization gap.
For background purposes, referring to FIGS. 1 and 2, shown is a model for explanation of the phenomenon occurring in the charging process via electrographic head 20 and paper recording medium 30. For clarity, only one nib 24 is shown but it can be appreciated that many nibs, positioned in a longitudinally extending array or nibline, are housed in head 20. Nib 24 is formed on substrate 22 or suspended in an insulating mold (not shown), and is connected to lead line 23 for supplying a charging voltage to nib 24. Air gap 27 exists between the end of nib 24 and the surface of recording medium 30 in order that the medium surface may be charged, or receive deposited charge. Medium 30 comprises a dielectric layer 32 deposited on a conductive base 34.
A pulsed voltage is applied between nib 24 and counter electrode or "backplate" 36. Because of the electric field concentrations during the voltage pulse via nib 24 at the edges of nib 24, there are field emissions 26 of electrons at the edges of nib 24. These electrons cause an ionization of air in gap 27. This ionization ignites a glow discharge in discharge region 28, near the central portion of nib 24 surrounded by field emission 26. The portion of gap 27 represented by discharge region 28 becomes ionized and therefore conductive. Discharge region 28 charges up the medium to a voltage where the voltage across the core gap drops to the glow discharge maintenance voltage. When the voltage drop reaches the glow discharge maintenance voltage, the discharge region 28 will be extinguished and the charge deposition on the surface of medium 30 will cease.
In electrographic printing, a non-uniformity or non-repeatability of the electrical discharge at the recording electrodes or nibs creates a problem in image quality and should be avoided. As a result of this discharge non-uniformity, the latent electrostatic image spots, or pixels, created on the recording medium are non-uniform in shape and may be enlarged or irregular in size compared to other latent image spots on the same page or line. This phenomenon is known in the art as "flare" or "flaring". Flare is detrimental to the image quality of printed or plotted images on the recording media because the "flared out", irregular dot patterns give the image an unattractive, irregular, speckled appearance.
The flared out dots are accompanied by an unusually large current pulse because of the large current needed to charge up the enlarged spot. Therefore, one way of reducing the size of flaring is to place limiting resistors in the electrode lead lines between the driving logic and the nib ends. Even with such limiting resistors in place, however, some flaring still occurs and spot size is still irregular and undesirable.
For further explanation of flaring, and referring especially to FIG. 1, shown is a single developed pixel 10 initially formed as a circular latent image spot by a single (in this case, wire) electrode or nib 24 (FIG. 2) and subsequently made visible with a conventional developer. Thus, the developed pixel 10 represents a visual appearance of the latent image spot. Pixel 10 is made up of core 12 and sometimes one or more nuclei 14 which surrounds core 12 and which are caused by field emission at the nib edges. Thus, nuclei 14, when they occur, are always formed around the perimeter of core 12. If the charge deposited at one of the nuclei 14 becomes excessive, there can develop a lateral electrical breakdown or spreading of this charge across the surface of dielectric layer 32 causing flares. Therefore, flare 16 usually appears as an enlargement of nuclei 14 resulting in a nonuniform developed image spot represented by the outer contour of pixel 10. In FIG. 1, core 12 is shown in cross hatch so as to distinguish from nuclei 14 but would normally be integrally developed with the remaining portion of developed pixel 10. Core 12 is charged in accordance with the writing voltage applied between nib 24 and counter electrode 36.
As seen in U.S. Pat. No. 4,801,919 to Hansen et al. which is assigned to a common assignee as this application, and is hereby incorporated by reference, flaring is reduced by incorporating a discharge quenching agent in the composition of the dielectric charge retentive layer of the electrographic recording medium. The patent also discloses coating a flaring suppressor agent on the surface of the dielectric layer of the electrographic recording media to enhance the charge retention or binding properties of the layer in order to suppress lateral discharge or charge spreading during recording. Although this method is effective, it may not always be practical or cost effective to treat the recording media with such an agent. Thus, further methods of reducing flaring are warranted.
As described above, transfer of electrostatic images onto a medium requires the movement of electrical charges through a gas in a gap between the electrostatic writing head and the recording medium. In the past, the gas in the gap has been ambient air. When the gap is relatively large (i.e. greater than approximately 8 microns) the mechanism of charge transfer can be explained on the basis of gaseous discharge phenomena. The Paschen curve, as seen in FIG. 3, describes the limiting conditions for charge transfer in this range. The Paschen curve is a plot of the minimum voltage which must be applied across a gap of thickness, d, filled with a gas at pressure, p, before glow-discharge breakdown will occur. As will be understood, approximately the voltage above the breakdown voltage is written on the medium as the electrostatic image.
A problem encountered while using air in the gap is that as the gap increases, the voltage needed for breakdown increases, thus, the amount of voltage available for forming the electrostatic image decreases. As the charge above the breakdown voltage decreases, the density of the resulting image decreases resulting in an image which is less desirable.
Other image quality problems resulting from electrographic printing have been addressed in the art. U.S. Pat. No. 3,979,759 to Simm and U.S. Pat. No. 4,030,106 to Bestenreiner et al. disclose a method and arrangement for eliminating undesirable density variations, or background noise, resulting from electrographic printing. Both patents discuss the use of nitrogen gas, or other inert gasses, blown through an opening in the housing containing the electrostatic print head for increasing the discharge current of a corona discharge. Xerox Disclosure Journal, Volume 14, Number 2, dated March/April 1989, entitled "A Gas Source For Improving Ionographic Output" suggests the use of pure nitrogen gas in a corona chamber of an ionographic machine. The addition of pure nitrogen into this configuration is used to increase ion generation efficiency thus resulting in better imaging. Although the above problems in image quality have been addressed, the above references utilizing Nitrogen do not address flaring or the prevention of irregular dot formation during electrographic printing.